Does it go into hybrid mode and use the heating elements?
You might be able to tell how much power it's using in the two modes,
by turning off everything else in the house, and watching your main power meter.
I've used mine and it's an accurate way to measure power use.
(It's what they are going to charge you for anyways)..
I have one of these meters,
Reading Itron Centron C1SR Digital Electric Meter - Watt emulator
and it's a snap to read the watt hours.
But, getting a baseline and using your bill should be fine too..